$ cat aa.c
#include <stdio.h>
                     
int main ()
{
    char c = 'A';

    char hexchars[] = "0123456789ABCDEF";
    char c_as_hex[] = {hexchars[c >> 4], hexchars[c & 0xF], '\0'};

    printf("%s\n", c_as_hex);
    return 0;
}
$ gcc -o aa aa.c
$ ./aa
41